1. Technical Domain
The invention relates to a laser beam read/write multilayer recording medium. It more precisely relates to the use of an alloy made up of germanium, indium, antimony and tellurium as active layer material for a re-writable optical disc.
2. Prior Art
Two types of optical re-writable discs are now currently in use. The first type involves the phase change of a solid material from a crystal phase to an amorphous phase and from the amorphous phase to the crystal phase. The second type involves the magneto-optical properties of some materials, in particular the Kerr effect polarisation rotation of a light beam.
The phase change optical discs have been the object of extensive researches for several years. They apply the principle following which it is possible to make a material shift from the amorphous state to the crystal state according to the duration and intensity of a laser beam applied to its surface. Moreover this method permits to directly overwrite new information on the information already recorded. The active layer, i.e. the recording layer, of a phase change disc storing information includes amorphous sites spread inside a crystal matrix, these amorphous sites thus being the recorded binary information. Reading this information is performed following an optical method, which consists in using a read laser beam generally guided by a groove. The read beam sweeps the surface of the disc while staying focused on its surface. A reflected beam is then obtained, which is directed onto a detection device. As the amorphous sites reflectivity is generally lower than the crystal zones reflectivity, it is then possible to discriminate whether an amorphous site or a crystal zone is detected and consequently to extract information represented by the amorphous sites.
The phase changing materials generally involved in this application are chalcogenides GeSbTe, AgTnSbTe or also InSbTe. Selenides and tellurium oxydes may also be used. The required properties for this materials are the following:
reversibility of two physical states (amorphous and crystalline),
stability of these two states at ambient temperature (from xe2x88x9240xc2x0 C. to +80xc2x0 C.),
an amorphisation time sufficiently low (about a few tens of nanoseconds),
a crystallisation time sufficiently low (about a few tens of nanoseconds),
a good stability in time and a good aptitude to endure cycling (defined as cyclability)
a melting point not too high (about 600xc2x0 C.).
These materials must be used in various conditions according to the kind of disc. For exemple, a CDxe2x88x92RW is used with a linear reading speed of 1.2 m/s, whereas a DVD-RAM is used with a linear reading speed of 6 m/s. It is then rare for a same material to be able to satisfy all these criteria, all the more since they are dependent. The minimum crystallisation time for example requires a particular composition.
During writing, the phase changing material is brought to its melting point, then endures a very quick annealing (about 10xc2x0 C. by nanosecond). For a correct operation, the active layer is encapsulated (sandwiched) between two layers of dielectric materials which do not inter-react with the phase changing material.
The active area of a recording medium of this type comprises generally a piling up of a transparent substrate, a first dielectric layer which proves inert with respect to the phase changing material, a layer of the said phase changing material, a second dielectric layer playing the same part as the first dielectric one and finally a layer intended for reflecting the reading beam, this layer being also a heat sink. Intermediate layers are interleaved with the layers listed above which are used as diffusion screens. This arrangement is currently involved in the production of CDxe2x88x92RW, DVDxe2x88x92RAM, DVDxe2x88x92RW and DVD+RW.
Recent developments have permitted to realise optical discs with two recording levels comprising two piling-ups such as the above-described one. These piling ups carry out the same functions but one of them is semi-transparent. It is thus possible to read and write through the semi-transparent piling up.
FIG. 1 is a schematic transverse cross-section of a two-level read/write recording optical disc using a laser beam following the prior art. This recording medium is described in the document EP-A-0 810 590. The recording medium comprises a first level 10 and a second level 20, separated by a spacer 3, the whole being sandwiched between a first transparent substrate 1 and a second substrate 2. The presented recording medium is intended for being written and read through the transparent substrate 1.
The first recording level 1, closer to the emitting source of a read or write laser beam comprises, superimposed onto the transparent layer 1, a dielectric layer 11, a phase changing material layer 12, an optical interference layer 13, a semi-transparent heat dispersion layer 14 and another optical interference layer 16.
The second recording level 2, farther from the emitting source of a read or write laser beam comprises, superimposed onto the spacer 3, a dielectric layer 21, a phase changing material layer 22, another dielectric layer 23 and a reflective layer 24 also used as a heat sink.
In a two-layer optical disc, the second level (the farther from the laser source) is read through the first level (closer to the laser source). A phase changing recording level comprises at least one active absorbing layer. The absorption rate of a thin absorbing layer is often defined by the optical absorption coefficient k. The optical index of this layer currently writes under complex form : N=nxe2x88x92jk, n being the refraction index of the layer. In first approximation, the energy absorbed by the phase changing material layer is proportional to exp(xe2x88x92ke) where e is the layer thickness. To favour transmission through the first level 1, it is the necessary to limit the coefficient k of the active material of the first level.
The problem which arises is to find a phase changing material which permits a good transmission of the laser beam intended for reaching the phase changing material of another level.
The already cited document EP-A-0 810 590 discloses a two-level optical recording medium. The first level phase changing material is an alloy whose composition is GexTeySbz, with 10 less than x less than 55, 45 less than y less than 55, and 38 less than z less than 48 and x+y+z=100%.
The document U.S. Pat. No. 5,254,382 discloses an optical recording medium which features a phase changing material layer made up of an alloy of germanium, indium, antimony and tellurium. This alloy has been retained so as to improve the durability and writing speed for an optical recording medium only featuring one layer. This document does not deal with the transparency of the phase changing material layer and the possibility of using this alloy on the first layer of a multilayer optical recording medium.
The document JP-A-11-126 366 discloses a phase changing optical recording medium whose material retained for the record layer permits to solve signal instability problems (currently named xe2x80x9cjitterxe2x80x9d). This material is an alloy from the family GeInSbTe.
The present invention brings up a solution to the exposed problem, i.e. using a phase changing material which favours the transmission of a laser beam directed onto a phase changing material layer of another level.
The object of the invention is then a multilayer recording medium with several read/write levels per laser beam, comprising a first semi-transparent level and at least a second level, the first level being closer to the emitting source of the laser beam, each level comprising a phase changing material with two states reversible under the action of the laser beam, characterised in that the phase changing material of the first level is an alloy whose formula is
[(GeyTe1-y)a(SbzTe1-z)1-a]1-b(In1-xTex)b
with:
0.4xe2x89xa6yxe2x89xa60.6
0.3xe2x89xa6zxe2x89xa60.5
0.4xe2x89xa6xxe2x89xa60.6
0.3xe2x89xa6axe2x89xa60.5
0.01xe2x89xa6bxe2x89xa60.3
The first level proving a reflectivity between 10% and 30%, a transmittance at least equal to 45%, a write power lower than 23 mW and an erase power lower than 10 mW.
Advantageously, the first level phase changing material layer is sandwiched between two confinement layer. Preferably, the two confinement layer are made up of materials such as ZnSxe2x80x94SiO2, SiO2, Si3N4 and GeN. Advantageously, the phase changing layer is at least 6 nanometres thick and the confinement layers are about 80 nanometres thick.